Transmission control method and device, and transmission apparatus

ABSTRACT

A transmission apparatus inserts unit signals into a stream transmitted from a first transmission path having a first bandwidth to a second transmission path having a second bandwidth. The insertion is controlled by an assignment signal that is generated by the following steps, which are repeated cyclically at intervals equivalent to the unit-signal length: a value representing the second bandwidth is added to a selected value; the sum is compared with a threshold; the assignment signal is set or reset according to the comparison result; the threshold value is subtracted from the sum to obtain a difference; and either the sum or the difference is selected, according to the assignment signal, as the selected value. This scheme enables bandwidth to be distributed evenly, thus reducing processing and memory requirements.

BACKGROUND OF THE INVENTION

The present invention relates to transmission apparatus of the generaltype that transmits a stream of unit signals and inserts further unitsignals into the stream, more particularly to a method of controllingthe points at which the further unit signals can be inserted.

Transmission apparatus of the above type is used in, for example,communication networks employing the asynchronous transfer mode (ATM).The unit signals in these networks are sequences of digital signalsreferred to as ATM cells, having a standard length of fifty-three bytes.

An example of an ATM transmission apparatus is shown in FIG. 1. Theapparatus 10 includes a plurality of interface units 11, 12, 13, 14 anda switching unit 15. Interface unit 11, for example, includes atransmission convergence (TC) layer 11A that terminates a transmissionpath 21 that may have any of a variety of interface specifications, andan ATM layer 11B that is interconnected to the switching unit 15.Interface unit 13 includes a TC layer 13A that terminates anothertransmission path 23 and an ATM layer 13B connected to the switchingunit 15. The other interface units have a similar structure (notvisible). In all there are m interface units, n of which are disposed onthe left side of the switching unit 15 in the drawing, where m and n arearbitrary positive integers (m>n).

The transmission paths 21, 23 may carry signals having various bit ratesor speeds. The parts of, for example, TC layer 11A that process thesesignals operate at corresponding speeds. The ATM layer 11B operates atthe fastest speed that might be encountered in the TC layer 11A. All ofthe ATM layers 11B, 13B, etc. have the same internal structure, so thata single type of ATM layer can be employed with transmission pathshaving different speeds just by changing the TC layer.

The term ‘bandwidth’ is often used as a synonym for bit rate or speed.Thus if TC layer 11A and ATM layer 11B are capable of processing signalsat rates of A bits per second and B bits per second, respectively, theywill be said to have bandwidths of A and B. In this case, A cannotexceed B (A≦B). If valid user cells are being transmitted at a rate of Cbits per second, the user cell traffic will be said to occupy abandwidth C, which must not exceed either A or B (C≦A≦B). Parts of thebandwidth B not occupied by user cells are filled with idle cells, andwith management cells used for the management of system resources.

The user cell traffic is controlled so that bandwidth C is distributedsubstantially evenly within the bandwidth B of the ATM layer 11B; thatis, user cells are kept moving through the ATM layer at a substantiallyeven rate. The reason for this is that if bandwidth C were to beconcentrated into one part of bandwidth B (if the user cell traffic wereto bunch up), then during the corresponding intervals of time, bandwidthB would be effectively filled, and large buffers would be required forbit-rate conversion from the ATM layer 11B to the TC layer 11A. An evendistribution of the user cell bandwidth C within the bandwidth B of theATM layer reduces the required buffer memory capacity of the apparatus.

The management cells mentioned above are inserted by the ATM layer. Aproblem that occurs in conventional ATM transmission apparatus is thatinsertion of these management cells can disturb the even celldistribution, causing the combined non-idle cell traffic to becomeoverconcentrated in certain parts of the ATM-layer bandwidth B. In theworst case, the user-cell bandwidth and management-cell bandwidth maytogether exceed the bandwidth capability A of the TC layer, forcing usercells to be dropped.

SUMMARY OF THE INVENTION

An object of the present invention is to insert management cells into anATM cellstream while maintaining an even distribution of assignedbandwidth.

A more general object is to insert further unit signals into a stream ofunit signals while maintaining an even distribution of unit signals inthe stream.

The invented method accordingly controls the insertion of first unitsignals into a stream of second unit signals transmitted from a firsttransmission path having a first bandwidth to a second transmission pathhaving a second bandwidth, by assigning points at which the first unitsignals may be inserted. The method comprises the steps of:

(a) adding a value representing the second bandwidth to a selected valueto obtain a sum value;

(b) comparing the sum value with a threshold value representing thefirst bandwidth to generate an assignment signal designating the pointsat which the first unit signals may be inserted;

(c) subtracting the threshold value from the sum value to obtain adifference value;

(d) selecting either the sum value or the difference value as theselected value, responsive to the assignment signal; and

(e) repeating steps (a) to (d) at unit intervals equal to the length ofthe first and second unit signals.

The invention also provides a transmission control device andtransmission apparatus using the invented method, including a firstarithmetic unit performing step (a), a comparator performing step (b), asecond arithmetic unit performing step (c), and a selector performingstep (d).

BRIEF DESCRIPTION OF THE DRAWINGS

In the attached drawings:

FIG. 1 is a block diagram of an ATM transmission apparatus;

FIG. 2 is a partial block diagram of an ATM transmission apparatusembodying the invention;

FIG. 3 is a more detailed block diagram of one of the control circuitsin FIG. 2;

FIGS. 4 and 5 are timing diagrams illustrating the operation of thecontrol circuit in FIG. 3; and

FIG. 6 illustrates a variation of the control-circuit structure in FIG.3.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the invention will be described with reference to theattached drawings, in which like parts are indicated by like referencecharacters.

Referring to FIG. 2, the embodiment is a transmission apparatus 46including a plurality of interface units 40, only one of which is shown.Each interface unit 40 has a TC layer 40A and an ATM layer 40B. The TClayer 40A includes a line signal-processing unit 41. The ATM layer 40Bincludes an ATM cell-processing unit 42, a quality management unit 43,and an internal interface processing unit 44. The ATM cell-processingunit 42 includes a control circuit (CC) 30B, the quality management unit43 includes a pair of cell insertion blocks (CIBs) 50A, 50B, and theinternal interface processing unit 44 includes a control circuit 30A.The internal interface processing unit 44 is coupled to a switching unit45. The line signal-processing unit 41 is coupled to an externaltransmission path 47.

Incoming signals from the external transmission path 47 are terminatedby the line signal-processing unit 41, which passes an ATM cellstream tothe ATM cell-processing unit 42. The ATM cell-processing unit 42 obtainssynchronization with the ATM cellstream and reformats the ATM cells tofacilitate processing by the other parts of the transmission apparatus46. The quality management unit 43 manages bandwidth and ensures thatquality-of-service requirements are met. The internal interfaceprocessing unit 44 interfaces the ATM cellstream to the switching unit45.

In the outgoing direction, the ATM cellstream from the switching unit 45is processed by the internal interface processing unit 44, then passedto the quality management unit 43, which again performs bandwidthmanagement and quality-of-service assurance functions. The ATMcell-processing unit 42 converts the ATM cells from the format usedwithin the apparatus to the standard ATM cell format. The linesignal-processing unit 41 generates the signals that send the ATMcellstream on the external transmission path 47.

To enable the quality management unit 43 to carry out its management andassurance functions, the cell insertion blocks 50A, 50B insert specialATM cells referred to herein as quality management cells in the incomingand outgoing cellstreams. These cells are also known as resourcemanagement cells, or simply as ‘management cells’ as in the backgrounddiscussion above. Control circuits 30A and 30B monitor the cellstreamsand decide where quality management cells may be inserted. This processis also a type of bandwidth management, and the control circuits 30A,30B may also be referred to as valid bandwidth management units.

Control circuit 30A generates a Cell Assignment signal indicating wherequality management cells may be inserted in the incoming cellstream.Control circuit 30B generates a Cell Assignment signal indicating wherequality management cells may be inserted in the outgoing cellstream. Thequality management unit 43 receives these Cell Assignment signals fromthe ATM cell-processing unit 42 and internal interface processing unit44 together with the incoming and outgoing cellstreams.

Both control circuits 30A, 30B have the internal configuration shown inFIG. 3, each comprising a selector 31, an adder 32, a comparator 33, asubtractor 34, a threshold control unit 35, and a valid cell detectionunit 36. These elements operate at unit intervals equal to one ATM cellinterval in the cellstream that passes through the quality managementunit 43. If, for example, the internal ATM cell format is fifty-fourbytes long, including the standard fifty-three bytes and a one-byteinternal switching tag, then the control circuits 30A, 30B operatecyclically with a cycle time equivalent to fifty-four bytes in thecellstream.

The cellstream entering the quality management unit 43 is denoted CS1 inFIG. 3; the cellstream leaving the quality management unit 43 is denotedCS2. The letters A and B denote values corresponding to the bandwidthsof the TC layer 40A and ATM layer 40B, respectively. The TC-layerbandwidth value A may be stored in a register (not visible) while thecontrol circuit is operating. The ATM-layer bandwidth value B may alsobe stored in a register, or may be hard-wired into the threshold controlunit 35.

The adder 32, functioning as the first arithmetic unit, adds theTC-layer bandwidth value A to a selected value (SV) received from theselector 31 and outputs the resulting sum AA.

The threshold control unit 35 receives the ATM-layer bandwidth value B,the Cell Assignment signal (CA), and a Valid Cell detection signal (VC).On the basis of these input values, the threshold control unit 35generates a Subtrahend-Threshold signal (ST). The ATM-layer bandwidth Bhas a fixed value corresponding to, for example, the maximum speed ofany external transmission path to which the transmission apparatus mightbe connected. The Valid Cell detection signal VC will be describedlater.

The comparator 33 compares the signals AA and ST output by the adder 32and threshold control unit 35 and generates the Cell Assignment signal(CA), using ST as a threshold value. The Cell Assignment signal CA issupplied to the relevant cell insertion block 50A or 50B in the qualitymanagement unit 43, and also to the selector 31. In the followingdescription, CA is a logic signal that is set at the high logic level,indicating that a quality management cell may be inserted, when AA isequal to or greater than ST, and is reset to the low logic level,indicating that a quality management cell may not be inserted, when AAis less than ST. The cell insertion block 50A or 50B is not forced toinsert a quality management cell when CA is high, but may do so ifnecessary.

Signals AA and ST are also supplied to the subtractor 34, whichfunctions as the second arithmetic unit. Using ST as a subtrahend, thesubtractor 34 subtracts ST from AA and outputs their difference DA. Thedifference may be positive or negative, so the difference signal DAincludes a sign bit.

The selector 31 has a control terminal (CT) that receives the CellAssignment signal CA, another input terminal, labeled ‘0’ in thedrawing, that receives the sum signal AA from the adder 32, and stillanother input terminal, labeled ‘1’ in the drawing, that receives thedifference signal DA from the subtractor 34. The selector 31 selects thesum signal AA when CA is low (‘0’), selects the difference signal DAwhen CA is high (‘1’), and outputs the selected signal as the SelectedValue SV.

The valid cell detection unit 36 monitors the cellstream CS1 enteringthe quality management unit 43 to detect valid user cells, that is,valid ATM cells addressed to a user, and generates the Valid Cellsignal. This signal is active when a valid cell is detected, and isinactive when, for example, an idle cell is detected.

In the quality management unit 43, cell insertion block 50A or 50Binserts quality management cells, if necessary, when permitted to do soby the Cell Assignment signal. The cellstream CS2 leaving the qualitymanagement unit 43 has the same speed as the entering cellstream CS1.The two cellstreams CS1, CS2 are normally identical except for thepresence in cellstream CS2 of quality management cells inserted by thecell insertion block 50A or 50B.

Two examples of the operation of the transmission apparatus 46 will begiven next. In both examples, the TC-layer bandwidth value A is ten(A=10), and the ATM-layer bandwidth value B is thirty-five (B=35).

Referring to FIG. 4, in the first example, no valid cells are detected,and the Subtrahend-Threshold signal ST remains fixed at a value equal tothe B (thus, ST=35). The cell numbers at the top of FIG. 4 indicateconsecutive cell periods or slots in the ATM cellstreams CS1, CS2. AllATM cells in cellstream CS1 are idle cells containing no valid userinformation.

The adder output AA and subtractor output DA both start at zero duringcell period zero. Since AA (zero) is less than ST (thirty-five), theCell Assignment signal CA is low during this period and the qualitymanagement unit 43 is not permitted to insert a quality management cellin the first cell slot E1 in cellstream CS2.

The low CA signal causes the selector 31 to select the AA signal, so inthe next cell period, the adder 32 adds A to AA, increasing AA from zeroto ten. This value is still less than the threshold value ST, so CAremains low, preventing a quality management cell from being inserted incell slot E2 in cellstream CS2. The subtractor 34 subtracts ST from AA,obtaining a DA value of minus twenty-five.

Similar operations continue in the following two cell periods, AA and DAincreasing by ten in each period. No quality management cell can beinserted in either of the corresponding cell slots E3 and E4 incellstream CS2. Cell slots E1, E2, E3, E4 all hold idle cells.

In cell period four, AA reaches forty, which exceeds the threshold valueST, so the CA signal goes high, enabling a quality management cell to beinserted in the corresponding cell slot V1 in cellstream CS2. If aquality management cell is waiting to be inserted at this time, it isinserted in this cell slot V1. The high CA signal causes the selector 31to select the DA signal, which is always thirty-five (ST) less than AAand now has the value five.

In cell period five, the adder 32 adds A (ten) to DA (five), obtainingfifteen as the new value of AA. This is less than the threshold ST, sothe CA signal goes low again. In cell period six, the selector 31selects the AA signal, to which the adder 32 adds the value of A, so AAincreases from fifteen to twenty-five. This is still less than thethreshold ST, so the CA signal remains low.

In cell period seven, AA reaches thirty-five, which is equal to thethreshold ST, so the CA signal goes high. If necessary, a qualitymanagement cell can be inserted in the corresponding cell slot V2 incellstream CS2. The high CA signal causes the selector 31 to select theDA signal, which is now equal to zero. In cell period eight, the adder32 adds A (ten) to this DA value (zero) and AA returns to the same value(ten) as in cell period one.

As long as no valid user cells are detected, the above operation repeatscyclically. Quality management cells can be inserted, if necessary, intwo out of every seven cell periods, at alternating intervals of threeand four cells: for example, in cell slots V3, V4, and so on. These cellslots are distributed as evenly as possible in cellstream CS2.

In effect, the TC-layer bandwidth A (ten) is being distributed evenly inthe ATM-layer bandwidth B (thirty-five) in FIG. 4. In the outgoingdirection, the cellstream that leaves the interface unit 40 and istransmitted on the external transmission path 47 includes only qualitymanagement cells and idle cells taken from the valid cell slots V1, V2,V3, V4, . . . in cellstream CS2.

Referring to FIG. 5, in the second example of the operation of thetransmission apparatus 46, cellstream CS1 includes a user cell.Accordingly, cellstream CS1 is shown, in addition to the other signalsthat were shown in FIG. 4. In this example, when the Valid Celldetection signal becomes active, the threshold control unit 35 addsbandwidth value B to the Subtrahend-Threshold value ST, increasing STfrom thirty-five to seventy (70=35+35).

In cell periods zero to four, no valid cell is detected, so VC remainsinactive and the same operations take place as in FIG. 4. In cell periodfour, the CA signal goes high and a valid cell slot V1 is designated.

In cell period five, the valid cell detection unit 36 detects a validuser cell U1 in cellstream CS1 and activates the Valid Cell signal. Theuser cell is passed into cellstream CS2, and will remain present in thecellstream output on the external transmission path 47.

When a user cell such as U1 is detected, since part of the TC-layerbandwidth (A) must be assigned to this cell, there is a sudden increasein the consumption of the TC-layer bandwidth A in the vicinity of theuser cell. This bandwidth is consumed regardless of the results ofoperations performed in the control circuit, because the user cell mustbe transmitted to the node (not visible) at the far end of the externaltransmission path 47.

To compensate for this consumption of bandwidth, the control circuitmust delay the next assertion of the Cell Assignment signal. It is forthis reason that, when the user cell is detected and the Valid Cellsignal VC becomes active, the threshold control unit 35 doubles the STvalue from thirty-five to seventy.

The AA value, which had returned from forty in cell period four to fivein cell period five, now increases in increments of ten over the nextseveral cell periods, while remaining below the new threshold value ofseventy. This threshold value is finally exceeded in cell period eleven,at which point the Cell Assignment signal CA goes high and a valid cellslot V2 is designated for possible insertion of a quality managementcell. This second valid cell slot occurs in the same position as thethird valid cell slot V3 in FIG. 4, reflecting the fact that a validcell slot has been consumed by the user cell U1 in the interim.Responding to the high Cell Assignment signal, the threshold controlunit 35 returns the Subtrahend-Threshold value ST to thirty-five.

Subsequent operations proceed as in FIG. 4. A third valid cell sot V3,equivalent to V4 in FIG. 4, is designated for possible insertion of aquality management cell in cell period fourteen.

User cells have priority over quality management cells, so if a usercell were to be detected in a cell period that had already beendesignated for possible insertion of a quality management cell, thecorresponding cell slot would be assigned to the user cell, even ifthere were a quality management cell waiting to be transmitted.

In the example in FIG. 5, if quality management cells are inserted inall of the available cell slots, they will outnumber the user cells, ofwhich there is only one, so that the TC-layer bandwidth A is occupiedprincipally by quality management cells. It is more common, however, foruser cells to outnumber quality management cells. In any case, as thedensity of user cells in the bandwidth increases, the density of validcell slots available for assignment to quality management cellsdecreases in compensation. Conversely, as the density of user cellsdecreases, the density of valid cell slots available for assignment toquality management cells increases. Under all conditions, the inventionoperates to maintain an even distribution of cell slots assignable toquality management cells and ensure that the insertion of qualitymanagement cells does not cause the sum of the bandwidth occupied byuser cells and the bandwidth occupied by quality management cells toexceed the TC-layer bandwidth A.

For this reason, in the outgoing direction, the cellstream can beconverted from the speed or bandwidth B of the ATM layer to the speed orbandwidth A of the TC layer without the need for a large buffer memory,and without the need to drop user cells.

In the incoming direction, the even distribution of bandwidth A inbandwidth B enables processing such as cell header conversion to beexecuted at a lower speed than if the cells assigned as part ofbandwidth A were to be concentrated in one part of bandwidth B; that is,if these cell were to be bunched up in the cellstream leaving thequality management unit 43. The necessary processing can therefore beaccomplished with less hardware.

If the TC layer 40A is altered, the associated bandwidth A may change,but no corresponding change in the control circuits in the ATM layer 40Bis necessary, except to change the A value supplied to the adder 32.This simplifies the manufacture and reduces the cost of the interfaceunit 40.

In a variation of the embodiment described above, theSubtrahend-Threshold value ST is kept constant, and the first arithmeticunit is modified to perform both addition and subtraction. The firstarithmetic unit performs addition as described above (A+SV=AA), andsubtracts the internal bandwidth B from the selected value SV, therebyreducing the sum AA of SV and A, when a user cell is detected. Thethreshold control unit 35 can then be eliminated. This variation isillustrated in FIG. 6. The first arithmetic unit is an adder/subtractor37 that replaces the adder 32 of FIG. 3.

In another variation, only control circuit 30A and cell insertion block50A have the invented configuration. The insertion of quality managementcells in the incoming direction, in which the bandwidth increases from Ato B, is controlled by conventional means. The insertion of qualitymanagement cells is unlikely to cause serious problems in this directionof increasing bandwidth.

In still another variation, the control circuit 30A is disposed in thequality management unit 43 instead of the ATM cell-processing unit 42.Alternatively, the control circuit 30A may be disposed in a locationoutside the interface unit 40. The insertion of quality management cellsmay also take place outside the interface unit 40.

Needless to say, the bandwidth values (A=10, B=35) in the embodimentabove were only illustrative; the invention can be practiced with anybandwidth values A and B (preferably with A≦B).

The invention is not limited to ATM transmission apparatus; it can beapplied to any type of transmission apparatus having an internaltransmission path with a bandwidth B that exceeds the external bandwidthA of the transmission line to which the apparatus is connected, in orderto distribute bandwidth A evenly within bandwidth B. The componentelements of the control circuit remain an arithmetic unit that adds thevalue of A to a selected value SV at certain intervals, a comparatorthat compares the resulting sum AA with a threshold value ST derivedfrom the internal bandwidth B, another arithmetic unit that subtractsthis threshold value ST from the sum AA to obtain a difference DA, aselector that selects either AA or DA as the selected value SV accordingto the comparator output, and preferably a detection unit that monitorsthe bandwidth B to detect parts that have already been assigned as partof the bandwidth A. The comparator output is used to assign other partsof the internal bandwidth B to the external bandwidth A.

The bandwidth assigned in accordance with the comparator output need notbe used for the insertion of quality management cells; other types ofcells may be inserted. More generally, the bandwidths A and B may bedivided into any type of fixed-length data units or unit signals, whichneed not be referred to as cells.

The invention can be practiced in either hardware or software.

Those skilled in the art will recognize that further variations arepossible within the scope claimed below.

1. A method of controlling the insertion of first unit signals into astream of second unit signals transmitted from a first transmission pathhaving a first bandwidth to a second transmission path having a secondbandwidth, by assigning points at which the first unit signals may beinserted, the first unit signals and the second unit signals having apredetermined length, the method comprising the steps of: (a) adding avalue representing the second bandwidth to a selected value, therebyobtaining a sum value; (b) comparing the stun value with a thresholdvalue representing the first bandwidth, thereby generating an assignmentsignal designating the points at which the first unit signals may beinserted; (e) subtracting the threshold value from the sum value,thereby obtaining a difference value; (d) selecting one of the sum valueand the difference value as the selected value, responsive to theassignment signal; and (e) repeating said steps (a) to (d) at unitintervals equivalent to the predetermined length.
 2. The method of claim1, wherein the first bandwidth is greater than the second bandwidth. 3.The method of claim 1, further comprising the steps of: (f) detectingthe presence of second unit signals having priority over the first unitsignals in the stream, before the insertion of the first unit signals;and (g) increasing the threshold value when one of the second unitsignals having priority over the first unit signals is detected.
 4. Themethod of claim 1, further comprising the steps of: (f) detecting thepresence of second unit signals having priority over the first unitsignals in the stream, before the insertion of the first unit signals;and (g) subtracting the threshold value from the selected value, therebyreducing the sum value, when one of the second unit signals havingpriority over the first unit signals is detected.
 5. The method of claim1, wherein the first unit signals and second unit signals areasynchronous-transfer-mode cells, and the unit intervals are cellperiods equivalent to an asynchronous-transfer-mode cell length.
 6. Themethod of claim 5, wherein the first unit signals areasynchronous-transfer-mode quality management cells and the second unitsignals include asynchronous-transfer-mode user cells.
 7. A transmissioncontrol device for controlling the insertion of first unit signals intoa stream of second unit signals transmitted from a first transmissionpath having a first bandwidth to a second transmission path having asecond bandwidth by assigning points at which the first unit signals maybe inserted, the first unit signals and the second unit signals having apredetermined length, the method comprising the steps of: a firstarithmetic unit adding a value representing the second bandwidth to aselected value, thereby obtaining a sum value; a comparator connectedthe arithmetic unit, comparing the sum value with a threshold valuerepresenting,the first bandwidth, thereby generating an assignmentsignal designating the points at which the first unit signals may beinserted; a second arithmetic unit connected to the comparator,subtracting the threshold value from the sum value, thereby obtaining adifference value; and a selector connected to the second arithmeticunit, selecting one of the sum value and the difference value as theselected value, responsive to the assignment signal; wherein the firstarithmetic unit, the comparator, the second arithmetic unit, and theselector operate at unit intervals equivalent to the predeterminedlength.
 8. The transmission control device of claim 7, wherein the firstbandwidth is greater than the second bandwidth.
 9. The transmissioncontrol device of claim 7, further comprising: a detection unitdetecting the presence of second unit signals having priority over thefirst unit signals in the stream, before the insertion of the first unitsignals, thereby generating a detection signal VC; and a thresholdcontrol unit connected to the comparator and the detection unit,increasing the threshold value when one of the second unit signalshaving priority over the first unit signals is detected.
 10. Thetransmission control device of claim 7, further comprising: a detectionunit connected to the first arithmetic unit, detecting the presence ofsecond unit signals having priority over the first unit signals in thestream, before the insertion of the first unit signals, therebygenerating a detection signal VC; wherein the first arithmetic unitsubtracts the threshold value from the selected value, thereby reducingthe sum value, when one of the second unit signals having priority overthe first unit signals is detected.
 11. The transmission control deviceof claim 7, wherein the first unit signals and the second unit signalsare asynchronous-transfer-mode user cells, and the unit intervals arecell periods equivalent to an asynchronous-transfer-mode cell length.12. The transmission control device of claim 11, wherein the first unitsignals are asynchronous-transfer-mode quality management cells, and thesecond unit signals include asynchronous-transfer-mode user cells. 13.The transmission apparatus including the transmission control device ofclaim 7 and a cell insertion block for inserting the first unit signalsin said stream, wherein the first transmission path is internal to thetransmission apparatus and the second transmission path is external tothe transmission apparatus.